Variable background intensity apparatus for imaging systems

ABSTRACT

A variable background intensity control system employs a modulator plate having different density or transmittance regions located thereon. The modulator contains an area upon which an aperture slit or image is normally registered. By varying the position of this image on the plate, one can achieve a change in background intensity. 
     Other embodiments show the use of additional aperture images registering on other regions of the modulator to achieve background or contrast variations.

BACKGROUND OF INVENTION

A new and useful technique for veiwing transparent or phase objects hasbeen described wherein a modulator apparatus comprising a photographicfilm or similar device having variable transmittance areas is positionedat a Fourier plane in the optical path of a compound microscope. Thetechnique and apparatus have generally been referred to as a modulationcontrast microscope.

In any event, the technical operation of such systems and variousembodiments as well as the theory of operation are the subject matter ofmy copending application entitled MODULATION CONTRAST MICROSCOPE, Ser.No. 476,518 filed on June 5, 1974 and MICROSCOPY SYSTEMS PARTICULARLYADAPTED FOR VIEWING TRANSPARENT OBJECTS filed on Sept. 5, 1974, Ser. No.503,394.

The advantages described and results obtained from such systems enableone to view phase or transparent objects as clear and as full of detailas such images produced by the more expensive and complicated phasecontrast and interference contrast systems.

The conversion of an ordinary compound microscope to one capable ofviewing typical objects is simple and inexpensive and basically requiresan aperture slit or a rectangular illuminating source positioned beforethe condenser lens and a modulator after the objective, which modulatoris positioned in a Fourier plane conjugate to the slit.

It would be be desireable to also provide means for varying thebackground intensity in such a system to enable a user to adjust thebackground intensity according to his preferences or according to thespecimen to be viewed.

It is therefore an object of the present invention to provide a variableaperture resulting in a variable background intensity control for amodulation contrast optical system such as a microscope.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENT

An optical system particularly adapted for use in microscopy and usefulfor viewing typical objects, said system including a source ofillumination, an aperture slit positioned above said source and meansincluding a condenser and objective lens adapted to focus the image ofsaid slit at a Fourier plane, the improvement therewith of apparatus forvarying the background intensity of a display associated with saidsystem comprising of a modulator plate positioned at said plane andhaving different transmittance regions on a surface thereof, with oneregion normally adapted to receive said image of said aperture slit andmeans coupled to said aperture slit to alter said position of said slitimage to cause at least a portion thereof to impinge on another regionof said modulator. While the word transparent object is used, one canview any object and hence is used in its broadest sense.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a top plan view of a modulator plate and useful in thisinvention.

FIG. 2 is a schematic diagram of a modulation contrast microscope foruse with this invention.

FIGS. 3A to 3C are schematic views useful in explaining the operation ofthe invention.

FIGS. 4A to 4C are top plan views of the modulator and aperture imagefor various background intensity controls.

FIG. 5A is a schematic view of a background control employing twoapertures.

FIG. 5B is a plan view of the apparatus shown in FIG. 5A.

FIG. 6 is a schematic view of a system for controlling the amount oflight passed through one aperture.

FIG. 7 is a plan view of one type of slit used in the invention.

FIG. 8 is a plan view of one type of a polarizer useful in practicingthe invention.

DETAILED DESCRIPTION OF DRAWINGS

Referring to FIG. 1, there is shown a typical modulator member 10 usedto alter the amplitude of light passing therethrough according to theprinciples of the modulation contrast microscope.

The modulator includes a first section 11 designated by the numeral D toindicate a dark region, a second strip 12 designated as G or a greyregion as compared to the darker region 11 and a region 13 designated asB or bright. The strip 12 is generally slightly offset from the centerof the modulator, but may be centered as well. The modulator 10 may befabricated from photographic film with the appropriate areas or sectionsas 11, 12 and 13 suitably exposed according to the transmittancerequired.

FIG. 2 shows a basic modulation contrast microscope as more particularlydescribed in the above noted copending applications.

The system includes a light source 14, as found in a conventionalmicroscope. A slit 15 is placed between the source 14 and the condenserlens 16. The slit and source may also be replaced by a rectangularconfiguration illuminating source such as a closely wound lamp filament.The specimen plane is shown as 17 and for example, may be a transparentobject. The objective lens 18 is located above the specimen plane 17 asin a conventional microscope. The modulator 10 (FIG. 1) is positionedabove the objective at the Fourier plane. The image of the sourceaperture or slit 15 is conjugate at the Fourier plane and is registeredwith a region on the modulator 10. Located above the modulator 10 is theimage plane 19.

A brief description of the operation of the system will be given.

The modulator 10 as located behind the objective 18 processes the lightgoing to the image, making optical gradients in the transparent specimen17 visible.

The image of the slit 15 is focused on the grey region (12 of FIG. 1) ofthe modulator 10 and represents the light passing through thenon-gradient portions of the object. Light passing through opticalgradients which are relatively perpendicular to the slit, will passthrough the clear or bright section (13 of FIG. 1) of the modulator.Light from optical gradients which face in the opposite direction passthrough the dark section (11 of FIG. 1) of the modulator 10.

When the light reaches the image plane 19, the gradients appear brightand dark respectively and hence, so does the object.

Because slopes in one direction are darker than slopes in the oppositedirection, the modulation contrast microscope image presents an illusionof three dimensions.

Typically, the tansmittance of the three sections are 100% for thebright section 13, 15% for the grey section 12 and 1% for the darksection 11. Other ratios may suffice as well. While, black, grey andwhite can be used, it is also noted and indicated that the sectionshaving the same transmittance ratios can be colored and hence the imageof the transparent object or specimen will appear in color.

The background intensity in the image plane of the modulation contrastmicroscope is controlled by the transmittance of the region of themodulator that passes the image of the source aperture or slit 15. Inthe above described example, this would be the grey region 12. The imagecontrast is proportional to the ratio of the background intensity andthe image intensity.

Assuming the above noted transmittances of 100%, 15% and 1%, for theBright (B), Grey (G) and Black (D) regions of the modulator 10 with theslit image registered at the Grey region, the background intensity willbe about 15% of the input intensity (Io). Thus, if one changes thetransmittance of the grey region, the background intensity will vary.

Thus, one could provide different modulators having differenttransmittances of Grey (G) to obtain background variation. It is alsounderstood that the Black region (D) may be located outside the opticalpath.

Referring to FIGS. 3A to 3C, another way of accomplishing backgroundintensity variation is to offset the image of the slit so that it passespartly through the grey region G and partly through another region.

FIG. 3A is a diagrammatic view showing the modulator 10 with the slitimage 20 and slit aperture 21 aligned centrally with the optic axis 25of the microscope. With the above described transmittance ratios, thebackground intensity would be 15% of the input intensity, (Io).

In FIG. 3B, the slit 21 is moved laterally to the right of the opticaxis 25. The image 20 of the slit now is positioned between the darkregion 11 and the grey region 12 of the modulator. Hence, in this casethe background intensity will be less than 15% of the input intensity(Io). The exact value determined by how far the slit or aperture 21 ismoved.

In FIG. 3C, the slit is moved to the left of the optical axis 25 tocause the slit image 20 to register between the grey area 12 and thebright area 13, thus causing the background intensity to be greater than15% of the input intensity (Io).

The slit can be moved laterally as shown by conventional known apparatus22 (FIG. 2) as by gear systems, a micrometer mechanism and so on, whichdevice or mechanism can be built into or added to the body of themicroscope. Or, again a modulator can be fabricated so that the centralstrip or grey region is offset accordingly to thereby cause the image ofthe slit 20, when central with the optic axis, to be positioned as shownin FIGS. 3B and 3C due to the layout of the modulator.

Referring to FIG. 4, there are shown three plan views of a modulator as10 of FIG. 1, with the slit image in the positions shown in FIGS. 3A to3C.

It is noted to provide clarity and consistency, the same referencenumerals have been retained to designate the like parts.

The intensity in the background of the image plane is proportional tothe area of the slit image and the transmittance of the region of themodulator through which it passes.

The general expression is

    I.sub.B = Δw(l)Io Tθ + (w-Δw)(l)Io T.sub.G

where

I_(B) = background intensity

Io = input intensity

Tθ = transmittance of either the dark region T_(D) or the bright regionT_(B)

w = width of the image slit

l = length of the image slit

Δw = distance that the image slit is moved out of the grey area

T_(G) = transmittance of grey region

Thus, if for example Δw = 1/2w then:

    I.sub.B = 1/2w(l)Io(0.01) + 1/2w(l)Io (0.15)

This is where the slit image is moved to the dark area and hence:

    Tθ = T.sub.D = 1% = 0.01

and

    T.sub.G = 15% = 0.15

therefore:

    I.sub.B = 1/2(wl)Io (0.15 + 0.01)

    I.sub.B = 1/2(wl)Io (0.16)

Thus the background intensity is less than 15% as in FIG. 4B. For therelation shown in FIG. 4C using the above equation where:

    Tθ = T.sub.B = 1

then:

    I.sub.B = 1/2(lw)Io (0.15 + 1.00)

or

    I.sub.B = 1/2(lw)Io (1.15)

and the background intensity is greater than 15%.

The registration of the slit can therefore vary from

Δw = 0 to Δw = w

The background intensity can also be varied by adding more slits orapertures that pass through the bright region 13 of the modulator 10,while leaving the original slit image 20 registered at the grey region12. However, the background intensity cannot be less than thetransmittance (T_(G)) of the grey region. However, as will be explained,symmetrical illumination is increased while coherence is reduced, thusapproaching bright field illumination.

As indicated above, the background intensity is controlled by thetranmission of the region that passes the image 20 of the slit.

Hence, as shown in FIG. 3A, if there is one slit and the slit apertureis aligned with the optical axis 25 to cause the slit image 20 to passonly through the grey area 12, then the background intensity is that ofthe grey region T_(G) or 15% of the input intensity Io.

In FIG. 5, two slits 21 and 30 are shown.

Slit 21 is at the center of the optic axis 25 as in FIG. 3A and hence,the slit image 20 associated with slit 21, is at the grey region 12. Anadditional slit 30 to the left of slit 21 causes an additional slitimage 31 to be registered on the bright section 13 of the modulator 10.Thus, the image 30 passes the bright section 13 (T_(B)) and will beadded to T_(G). The expression is as follows:

    I.sub.B = Io[(lw).sub.1 T.sub.G + (lw).sub.2 TθN]

where:

I_(B) = background intensity

Io = input intensity

(lw)₁ = product of length and width of aperture image 20.

(lw)₂ = product of length and width of aperture image 31

Tθ = T_(B) or T_(D)

n = number of apertures

If the amount of light that passes through slit 30 were controlled, theadded intensity due to the slit image 31 can be varied from zero to one.

A technique for controlling the amount of light passing through slit 30is shown in FIG. 6.

A first polarizer plate 35 is positioned between the source and the slitor apertures 30 and 21. The image intensity Io passes through plate 35prior to passing through slits 21 and 30. A second plate or polarizer 36is positioned between the modulator 10 and the slits 30 and 21 and lightpassing through the slits is passed through polarizer 36.

If the polarizers 35 and 36 are parallel or oriented for maximumtransmission, then both slits 21 and 30 will pass light, assuming aunity transmission for each polarizer.

However, if the polarizers 35 and 36 are crossed, no light passesthrough slit 30, but light passes through slit 21 since the plate 36 hasa clear area or aperture located thereon congruent with the slitaperture 21. Hence, light always passes through aperture 21 to form theslit image 20. However, with crossed polarizers, light will not passthrough the polarizer 36 and hence, the image 31 will not be providedfor crossed polarizers 35 and 36.

Crossed polarizers as 35 and 36 are well known in the art and examplesof polarizer rotation to control light transmittal are many and found inconventional text books.

In any event, by using the polarizers 35 and 36, as shown the intensityof light emanating from slit 30 can be controlled from a maximum to aminimum. The general expressions for the apparatus of FIG. 6 are asfollows:

    I background ˜ Io(lw).sub.1 T.sub.G + Io(lw).sub.2 T.sub.B cos.sup.2 α

where

α = the angle of rotation between parallel polarizers 35 and 36.

when

α = 0° then:

    I = Io[(lw).sub.1 T.sub.G + (lw).sub.2 T.sub.B ]

when

α = 90° then:

    I = Io(lw).sub.1 T.sub.G

as can be seen, when α = 90°, then the background intensity is equal to15% of the input intensity or that condition as obtained from theapparatus shown in FIGS. 3A and 4A.

Since the contributions of intensity of both slits as 21 and 30 accountfor the change in background intensity and since slit 30 is to registerin the bright side of the modulator, this slit can be of any shape aslong as its image falls within the desired modulator area.

Referring to FIG. 7, there is shown one arrangement for slits 20 and 31,which can be implemented. It is understood that other arrangements canbe used as evidenced by the above description and formats.

An offset slit as 21 is preset in a plate or thin planar member and ispositioned offset with the optical axis of the microscope.

An annular ring 30 corresponds to a slit as 30 of FIG. 6 and can be usedin lieu of a separate slit adjacent slit 21.

FIG. 8 shows a polarizer plate as 36 of FIG. 6, having an aperture 41cut out and relatively congruent with the slit aperture 21 such that itregisters with the slit 21. The annulus, as evidenced by slit 30, isshown on the polarizer plate as 40; to note that the polarizer plate ofFIG. 7A covers this portion of the annular aperture 30.

The plate as 35 is positioned as shown in FIG. 6 and may be rotated orcrossed, thus controlling the amount of light passed by the annularaperture 30. The following relation is determinative of the intensity:

    I = (AREA).sub.1 Io T.sub.G + (AREA).sub.2 Io T.sub.B cos.sup.2α

where

Area₁ = li × w₁

Area₂ = area of annular ring

When α = 0° the background intensity approximates a symmetrical annulusof which background intensity is of the type approaching bright fieldillumination.

Besides the above described polarizers, one could also accomplishbackground intensity variation by mechanically occluding the slits oruse variable neutral density wedges.

It is also noted that the sensitivity of the system is reduced as theslit width is reduced assuming that the grey region 12 of the modulatorstays the same. Hence, it has been determined that a variable slit widthserves to modify sensitivity. Therefore, if the width of the image slitis made less than the width of the grey region, then one can move theslit a predetermined amount before one sees a change in backgroundintensity or contrast.

This feature can be used to advantage for viewing certain types oftransparent specimens.

While certain techniques have been described for varying backgroundintensity, others will become apparent to those skilled in the art asthe mathematics and theory of operation have been clearly implementedand explained.

All such modifications are deemed to be within the spirit and scope ofthe invention as more particularly determined by the claims appendedhereto.

I claim:
 1. In an optical system particularly adapted for use inmicroscopy and useful for viewing typical objects, said system includinga source of illumination, an aperture slit positioned above said sourceand means including a condenser and objective lens adapted to focus theimage of said slit at a Fourier plane, the improvement therewith ofapparatus for varying the background of a display associated with saidsystem, comprising:a. a modulator plate positioned at said plane andhaving different transmittance regions on a surface thereof, with oneregion normally adapted to recieve said image of said aperture slit, andb. means coupled to said aperture slit to alter said position of saidslit image to cause at least a portion thereof to impinge on anotherregion of said modulator, whereby said background intensity is variedaccording to the portion of said slit image impinging on said anotherregion as compared to the portion of said image impinging on said oneregion.
 2. The optical system according to claim 1 wherein saidmodulator comprises a relatively circular plate containing a relativelycentral stripe region of a given transmittance, and a second region ofanother transmittance to the left of said stripe and a third region ofstill another transmittance to the right of said stripe.
 3. The opticalsystem according to claim 2 wherein said central stripe is a grey regionhaving a transmittance of about 15% of one of said other regions.
 4. Theoptical system according to claim 2 wherein said region to the left ofsaid stripe has a transmittance of between 0 to 2 percent of one of saidother regions.
 5. The optical system according to claim 2 wherein saidregion to the right of said stripe clear having a transmittance between80 to 100 percent.
 6. A method for varying the background intensity in amodulation contrast microscope comprising the steps of:laterally movingthe aperture image of said microscope between a first and second regionof a modulator plate to cause said image to register partly on saidfirst region of said plate and partly on said second region, to causethe background intensity to vary according to the portion of the area ofthe image registered on each region.
 7. In combination with a modulationcontrast microscope of the type employing a source for providing a beamof light for illuminating an object, a condenser lens for concentratingthe beam on the object position, an objective lens focused on the objectposition for receiving the beam after it has left the object, amodulator located at the Fourier transform plane and having at least twoadjacent regions of substantially different transmittance fordistributing light passing therethrough at an image plane, theimprovement therewith of apparatus for varying the background intensityat said image plane, comprising:a. a planar member having a first andsecond aperture on a surface thereof and positioned between said sourceand said condenser lens, one of said apertures positioned in apredetermined location with respect to the optic axis of said microscopeto cause an image from said aperture to register at a first region ofsaid modulator, and said other aperture positioned laterally from saidfirst aperture to cause an image from said second aperture to registeran another adjacent region of said modulator of a substantiallydifferent transmittance than said first region, and means for varyingthe intensity of light passing through at least one of said apertures ascompared to the intensity passing through said other aperture.
 8. Themicroscope according to claim 7 wherein said first aperture isrelatively rectangular in shape and has a width such that the imagetherefrom is relatively equal to the width of said first region of saidmodulator.
 9. The microscope according to claim 7 wherein said secondaperture is generally annular in shape.
 10. The microscope according toclaim 7 further including means located in the optical path of saidmicroscope and adapted to control the intensity of light passing throughsaid second aperture.
 11. The microscope according to claim 10 whereinsaid means include at least one polarizing plate positioned between saidplanar member and said condenser lens.
 12. An optical systemparticularly adapted for use in microscopy and useful for viewingobjects on a different intensity background with the aid of a lightbeam, comprising:a. means having discrete different density regionslocated at a first plane in the optical path of said system, said firstplane being relatively perpendicular to and positioned to intercept saidlight beam for modification by said means of the amplitude of said lightbeam relatively about a given region in both a greater and lesserintensity to thereby alter portions of the amplitude of said object'sphase gradients, b. an illumination source positioned in a differentplane conjugate to said first plane for illuminating said object, saidillumination source having a relatively planar image to provide anillumination pattern capable of being registered at said given region ofsaid means, and c. adjustable means for laterally moving saidillumination source to cause said image to register at both said givenregion and one of said other different density regions to vary thebackground intensity of said object as viewed.
 13. The optical systemaccording to claim 12 wherein said illumination source is of the typeemploying a rectangular filament.
 14. The optical system according toclaim 12 wherein said illumination source comprises a light source andan aperture plate having located thereon a slit of a width generallydetermined in accordance with the width of said given region.
 15. Amicroscope comprising means for supporting an object at an objectposition, an illumination source positioned at a given plane conjugateto a Fourier plane and having a predetermined image pattern, condensermeans responsive to said image pattern of said illumination source, anobjective focused on the object position for receiving said imagepattern after leaving the object, means for displaying the real image,means having different density regions located at the Fourier transformplane conjugate to said given plane for modification by said means ofthe amplitude of said light rays relatively about a given region locatedon said means in both a greater and lesser amplitude, whereby when anobject with phase gradients is examined, said means for displaying animage provides a display of said object with viewable contrast effects,means located in said optical path, said means including a first and asecond aperture, with said first aperture having an image patternregistered at one of said different density regions and said secondaperture having an image pattern registered at said given region andmeans for selectively varying the intensity of light of one of saidimage patterns with respect to said other to thereby alter thebackground intensity of said real image as displayed.
 16. A microscope,comprising in combination:a. an illumination source having a relativelyrectangular light pattern and positioned at a predetermined plane, b.means for focusing said light pattern at a second plane designated as aFourier transform plane and characterized in that spatial frequencies ofan object and relative maximum energy for each point on the gradient ofthe object are distributed, c. a modulator comprising a central regionof a given transmittivity and two adjacent regions thereto of adifferent transmittivity from each other and said central region, saidmodulator positioned at said Fourier transform plane and operative toalter the amplitude of said light pattern from an object according tosaid given transmittivity regions, d. means for displaying said alteredamplitude light pattern to obtain a view of said object, and e. meanscoupled to said illumination source to shift the same to cause saidrectangular pattern to impinge both on said central region and oneadjacent region to cause a change in background intensity at said meansfor displaying said altered amplitude light pattern.
 17. The microscopeaccording to claim 16 wherein said means for shifting said light patterncomprises a plate having an aperture and means for moving said platelaterally.
 18. In a modulation contrast microscope of the type having amodulator plate located at a Fourier plane, said plate having arelatively central region of a first transmittivity and a secondadjacent region of a second transmittivity at one side and a thirdadjacent region of another transmittivity at the other side, saidmicroscope normally having an aperture image which is registered at saidcentral region, in combination therewith of apparatus for varying thebackground intensity, comprising:a. means adapted to shift said apertureimage laterally to cause said image to register on both said centralregion and one other region, the amount of illumination from said imageon each region being determinative of the background intensity.